Kidnappings and attacks on aid convoys had been on the rise in Congo at the time of the assault on Attanasio and his entourage.
An Italian judge has ruled that two employees of a UN agency cannot be tried over the deaths of the Italian ambassador to the Democratic Republic of Congo (DRC), his bodyguard and a driver in 2021 because they have diplomatic immunity.
Luca Attanasio, bodyguard Vittorio Iacovacci and their driver Mustapha Milambo were killed during a botched kidnapping on a road in Eastern Congo as they were heading toward a World Food Programme (WFP) project.
Rome prosecutors had requested a trial for Rocco Leone and Mansour Luguru Rwagaza, who at the time of the attack were respectively the deputy chief of the WFP in Congo and a security officer, accusing the pair of negligence in organising the trip.
However, Judge Marisa Mosetti ruled on Tuesday that as employees of the United Nations, the two men enjoyed diplomatic immunity and cannot therefore be tried.
The prosecutors said they would appeal.
A DRC military court last year sentenced six men to life in prison for the killings.
Eastern Congo has been beset by violence for decades as rival militias fight government troops and each other for control of land and resources.
Kidnappings and attacks on aid convoys had been on the rise at the time of the assault on Attanasio and his entourage.